I. REVIEW OF MICROBIAL GENETICS
C. DNA REPLICATION IN EUKARYOTIC CELLS AND THE EUKARYOTIC CELL CYCLE
Fundamental statements for this learning object:
1. During DNA replication, each parent strand acts as a template for the synthesis of the other strand by way of complementary base pairing.
2. Complementary base pairing refers to DNA nucleotides with the base adenine only forming hydrogen bonds with nucleotides having the base thymine (A-T). Likewise, nucleotides with the base guanine can hydrogen bond only with nucleotides having the base cytosine (G-C).
3. Each DNA strand has two ends. The 5' end of the DNA is the one with the terminal phosphate group on the 5' carbon of the deoxyribose; the 3' end is the one with a terminal hydroxyl (OH) group on the deoxyribose of the 3' carbon of the deoxyribose.
4. To synthesize the two chains of deoxyribonucleotides during DNA replication, the DNA polymerase enzymes involved are only able to join the phosphate group at the 5' carbon of a new nucleotide to the hydroxyl (OH) group of the 3' carbon of a nucleotide already in the chain.
5. While the two strands of DNA are complementary, they are oriented in opposite directions to each other. One strand is said to run 5' to 3'; the opposite DNA strand runs antiparallel, or 3' to 5'.
6. Unlike the circular DNA in prokaryotic cells that usually has a single origin of replication, the linear DNA of a eukaryotic cell contains multiple origins of replication.
7. Because DNA can only be synthesized in a 5' to 3' direction and all DNA polymerase requires a primer, the ends of the linear eukaryotic DNA strands, called telomeres, have short, repetitive, noncoding DNA base sequences. A unique enzyme called telomerase binds to the telomeric DNA at the 3' end. The telomerase contains a small RNA template as a cofactor which is copied by DNA nucleotides to extend the 3' end. Once the extension is long enough, primase can assemble a short RNA primer on the lagging strand and DNA replication can proceed in a manner similar to the lagging strand of prokaryotic DNA.
8. Once the chromosomes have replicated, the nucleus divides by mitosis.
9. During interphase, cellular organelles double in number, the DNA replicates, and protein synthesis occurs. The chromosomes are not visible and the DNA appears as uncoiled chromatin.
10. During G1 phase, the period that immediately follows cell division, the cell grows and differentiates and new organelles are made.
11. DNA synthesis (chromosome replication) occurs during S phase.
12. During G2 phase, molecules that will be required for cell replication are synthesized.
13. Nuclear division is referred to as mitosis while cytoplasmic division is called cytokenesis.
14. During prophase, the chromatin condenses and the chromosomes become visible, the nucleolus disappears, the nuclear membrane fragments, and the spindle apparatus forms and attaches to the centromeres of the chromosomes.
15. During metaphase, the nuclear membrane fragmentation is complete and the duplicated chromosomes line up along the cell's equator.
16. During anaphase, diploid sets of daughter chromosomes separate and are pushed and pulled toward opposite poles of the cell.
17. During telophase, the nuclear membrane and nucleoli reform, cytokinesis is nearly complete, and the chromosomes eventually uncoil to chromatin.
18. During cytokinesis, the dividing cell separates into two diploid daughter cells.
DNA is a long, double-stranded, helical molecule composed of building blocks called deoxyribonucleotides (def). A deoxyribonucleotide is composed of 3 parts: a molecule of the 5-carbon sugar deoxyribose, a nitrogenous base (def), and a phosphate group.
To synthesize the two chains of deoxyribonucleotides during DNA replication, the DNA polymerase enzymes involved are only able to join the phosphate group at the 5' carbon of a new nucleotide to the hydroxyl (OH) group of the 3' carbon of a nucleotide already in the chain. The covalent bond that joins the nucleotides is called a phosphodiester bond (def). Each DNA strand has what is called a 5' end and a 3' end. This means that one end of each DNA strand, called the 5' end (def) , will always have a phosphate group attached to the 5' carbon of its terminal deoxyribonucleotide (see Fig. 1). The other end of that strand, called the 3' end (def) , will always have a hydroxyl (OH) on the 3' carbon of its terminal deoxyribonulceotide.
During DNA replication, each parent strand acts as a template for the synthesis of the other strand by way of complementary base pairing. Complementary base pairing (def) refers to DNA nucleotides with the base adenine only forming hydrogen bonds with nucleotides having the base thymine (A-T). Likewise, nucleotides with the base guanine can hydrogen bond only with nucleotides having the base cytosine (G-C). (In the case of RNA nucleotides, as will be seen later, adenine nucleotides form hydrogen bonds with nucleotides having the base uracil since thymine is not found in RNA.) As a result of this bonding, the DNA assumes its helical shape. Therefore, the two strands of DNA are said to be complementary. Wherever one strand has an adenine-containing nucleotide, the opposite strand will always have a thymine nucleotide; wherever there is a guanine-containing nucleotide, the opposite strand will always have a cytosine nucleotide (see Fig. 2).
While the two strands of DNA are complementary, they are oriented in opposite directions to each other. One strand is said to run 5' to 3'; the opposite DNA strand runs antiparallel, or 3' to 5' (see Fig. 1).
We will now look at how DNA is replicated in eucaryotic cells.
DNA Replication in Eukaryotes and the Eukaryotic Cell Cycle
As in prokaryotes, the linear chromosomes of eukaryotes replicate by strand separation and complementary base pairing (def) of free deoxyribonucleotides (def) with those on each parent DNA strand (see Fig. 4 and Fig. 5). As with prokaryotes, DNA replication in eukaryotic cells is bidirectional. However, unlike the circular DNA in prokaryotic cells that usually has a single origin of replication (see Fig. 2), the linear DNA of a eukaryotic cell contains multiple origins of replication (see Fig. 11).
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by Gary E. Kaiser, Ph.D. Last updated: August, 2019 |
As discussed earlier under prokaryotic DNA replication, DNA can only be synthesized in a 5' to 3' direction (see Fig. 5) and all DNA polymerase (def) requires a primer. To solve this problem, the ends of the linear eukaryotic DNA strands, called telomeres (def), have short, repetitive, noncoding DNA base sequences. A unique enzyme called telomerase binds to the telomeric DNA at the 3' end. The telomerase contains a small RNA template as a cofactor which is copied by DNA nucleotides to extend the 3' end. Once the extension is long enough, primase can assemble a short RNA primer on the lagging strand and DNA replication can proceed in a manner similar to the lagging strand of prokaryotic DNA.
Once the chromosomes have replicated, the nucleus divides by mitosis (def) (see Fig. 12 through 16).
The eukaryotic cell cycle is divided into two major phases: interphase and cell division.
A. Interphase
Ninety percent or more of the cell cycle is spent in interphase. During interphase, cellular organelles double in number, the DNA replicates, and protein synthesis occurs. The chromosomes are not visible and the DNA appears as uncoiled chromatin.
Interphase in a plant cell: see Fig. 17
Interphase in an animal cell: see Fig. 18
Interphase is divided into the following stages: G1, S, and G2.
1. G1 phase
During G1 phase, the period that immediately follows cell division, the cell grows and differentiates. New organelles are made but the chromosomes have not yet replicated in preparation for cell division.
2. S phase
DNA synthesis occurs during S phase. The chromosomes replicate in preparation for cell division.
3. G2 phase
During G2 phase, molecules that will be required for cell replication are synthesized.
B. Cell Division
Cell division consists of nuclear division and cytoplasmic division. Nuclear division is referred to as mitosis while cytoplasmic division is called cytokenesis.
1. Mitosis (nuclear division)
Mitosis is the nuclear division process in eukaryotic cells and ensures that each daughter cell receives the same number of chromosomes as the original parent cell. Mitosis can be divided into the following phases: prophase, metaphase, anaphase, and telophase.
a. Prophase
During prophase, the chromatin condenses and the chromosomes become visible. Also the nucleolus disappears, the nuclear membrane fragments, and the spindle apparatus forms and attaches to the centromeres of the chromosomes.
Prophase in a plant cell: see Fig. 19 and Fig. 20
Prophase in an animal cell: see Fig. 21 and Fig. 22
b. Metaphase
During metaphase, the nuclear membrane fragmentation is complete and the duplicated chromosomes line up along the cell's equator.
Metaphase in a plant cell: see Fig. 23
Metaphase in an animal cell: see Fig. 24
c. Anaphase
During anaphase, diploid sets of daughter chromosomes separate and are pushed and pulled toward opposite poles of the cell. This is accomplished by the polymerization and depolymerization of the microtubules that help to form the spindle apparatus.
Anaphase in a plant cell: see Fig. 25 and Fig. 26
Anaphase in an animal cell: see Fig. 27
d. Telophase
During telophase, the nuclear membrane and nucleoli reform, cytokinesis is nearly complete, and the chromosomes eventually uncoil to chromatin. Usually cytokinesis occurs during telophase.
Telophase in a plant cell: see Fig. 28 and Fig. 29
Telophase in an animal cell: see Fig. 30
2. Cytokinesis (cytoplasmic division)
During cytokinesis, the dividing cell separates into two diploid daughter cells. In animal cells, which lack a cell wall and are surrounded only by a cytoplasmic membrane, microfilaments of actin and myosin attached to the membrane form constricting rings around the central portion of the dividing cell and eventually divide the cytoplasm into two daughter cells. In the case of plant cells , which are surrounded by a cell wall in addition to the cytoplasmic membrane, carbohydrate-filled vesicles accumulate and fuse along the equator of the cell forming a cell plate that separates the cytoplasm into two daughter cells.
Last updated: Feb., 2020
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